Introduction to the Aristolochiales

Birthworts and Dutchman's pipes

The Aristolochiales are a group of paleoherbs, a basal group of
flowering plants. There are about
600 to 700 species, most of them in the genus Aristolochia (ca. 500 spp.),
and most of the rest in Asarum (ca. 100 spp.).

The majority of the Aristolochiales are tropical, though a number of them
range as far north as Canada, Scandinavia, and northern Japan. They may grow
as climbing vines, as short creeping herbs, and a few are shrub-like.

Members of this group have attractive leaves and flowers, often with bizarre
colors or patterns, and so they are cultivated in gardens. Some oddly shaped
flowers have been given names like "bird's head" and "Dutchman's pipe".
An example of such a plant is Aristolochia gnaxima, a climbing vine
shown at right. In this genus, the sepals are united into a long curving tube
which completely surrounds the reproductive structures. Often this tube is
mottled or dark-colored, producing a pungent odor that attracts pollinating flies.

Asarum : There are about 100 species of Asarum, today mostly
found in temperate regions of North America, Europe, and Asia. Most of these
are small creeping herbs with inconspicuous flowers and vestigial petals; the
colorful reddish sepals fulfill the role of petals.

Of the living Aristolochiales, only the Chinese Saruma henryi has fully developed petals.
Among the remaining genera, vestigial petals are found in some species of
Asarum, but the other taxa lack them altogether. In the absence
of petals, the sepals have become modified, often with dark colors
and unusual shapes. It is thus the sepals that attract pollinating insects.

In addition to the foul-smelling species of Aristolochia that attract
flies to pollinate them,
other species are important for the survival of butterflies. There
has been concern in India (Bhattacharya 1986) that since Aristolochia
indica has become endangered, the swallowtail butterfly Tros aristolochae may also be endangered. This butterfly will only lay its eggs on
Aristolochia vines, and its caterpillars make defensive use of the
toxic chemicals they ingest. In fact most swallowtail caterpillars in the
tribe Troidini feed on Aristolochiales foliage (Thorne 1996).

Image of Aristolochia gnaxima by Nan Crystal Arens; photos of Aristolochiales fossils by Brian R. Speer.
Image of Asarum caudatum flower provided courtesy of the Jepson Herbarium, all rights reserved.
Image of living Asarum plant provided by Virtual Foliage at the University of Wisconsin, and used by permission.